Apple apparently thinks that you’d prefer not to see the icons for your Mac’s built-in hard drive or those of any external drives on the desktop of macOS Mojave (and most of its predecessors). However, I want those icons on my desktop. Here’s how to make ‘em visible:
Click on the desktop so the Finder is “active.”
Select Preferences under the Finder menu (it’s at the top left hand side of the screen).
You’ll see a list of items on your desktop. Check the ones (hard disks, external disks; CDs, DVS, and iPods; connected services) you want to be visible.
Voila. Icons for those items now appear on your Mac’s screen.
(This how-to is based on my experiences and info on Apple's support pages — where the images sometimes come from.)